As long as you give him a diaper to poop in he will continue to do this and not learn. He's old enough to go cold turkey. Just prepare yourself for lots of accidents and when he asks for the diaper take him to the potty. Is he afraid of the big potty? Use a little training one.
My 2 year old refers to pooping as "squeezing". It helped to give it a name so I could encourage her to do it, rather then just sit on the potty waiting for it to happen on its own. She wants me to sit next to her before nap and bed so she can go poop. She can do it alone, but its some quiet together time that we don't usually get during the day. If she gets distracted I can remind her, are you squeezing? Then she refocuses and either says she's done or tries again. She's so excited about being able to accomplish this. Yes she still has a few accidents, but its nearly a thing of the past. I wish I'd learned something like this for my oldest, it took her a really long time to learn and she was the type that pooped every 4 days already. Peeing is so much easier, you don't have to work, lol. Its important for them to learn to sit still and do it.
Be sure he's getting lots of fiber in his diet! White rice/pasta/bread will constipate anyone, and makes pooping more of a chore. Lots of fresh fruits and veggies, and limit the processed carbs, and he should have more opportunities and it will be easier to accomplish.
Best wishes! I know this is tough, but get rid of those diapers - its unlikely he'll suddenly start pooping in the toilet. :) You can do it Mama!